If Sakurai thinks he's being biased towards modern Kirby content, then by all means, I want him to be even MORE biased.
Saying that Sakurai's bias is adding new content is a little too broad, especially if you're discussing Smash 4. Compare the treatment Donkey Kong got in Smash to KI. Both were in a fairly similar position back during the beginning of Smash 4's development. Both had made a recent comeback (DKCR 2010 and KI:U 2012), and the Smash fanbase vocalized wanting more content from both series. One series got milked, the other got curb stomped.
Now I can't get mad at Sakurai for not including a third DK newcomer in Smash 4, as Returns didn't really feature anyone aside from DK and Diddy (I guess Cranky would've worked, but I can see why Sakurai may have hesitated. Cranky probably needed a larger role like he had in TF). Regardless, Sakurai had plenty of material to work with to represent the DK series in other ways. The series only got a single new stage, no Assist Trophies, no items, a handful of trophies, and two remixes. Meanwhile, KI got two new stages, two Assist Trophies, about five or six items, an insane amount of trophies in the 3DS version, and four remixes.
Obviously Sakurai wants to add more content, but he doesn't always distribute the new content fairly. I know it's difficult to show blatant attention to every franchise, but the lack of love given to the iconic DK series after an incredible game like Returns is just abysmal. Seeing a niche series like KI get almost three times as much new content just adds insult to injury.